A 47-percent plurality of likely primary voters have a favorable impression of Lugar, first elected to the Senate in 1976. Thirty-one percent of likely primary voters have an unfavorable opinion of him. Mourdock remains relatively unknown -- even among likely primary voters. Just 24 percent of primary voters have a favorable opinion of Mourdock, while 18 percent have an unfavorable opinion of him. Notably, the sample of likely primary voters was drawn exclusively among Republicans and independents. Though the primary is open to all voters (Indiana voters do not register with a political party), those respondents who identified as Democrats were not asked if they intended to vote in the GOP primary. Just this week, according to exit polls, 11 percent of Wisconsin GOP presidential primary voters identified as Democrats. Strictly among voters who identified as Republicans, Lugar and Mourdock were virtually tied. The Senate primary will be held the same day as the state's presidential primary. The poll shows former Sen. Rick Santorum, R-Pa., and former Gov. Mitt Romney leading the race, with Santorum at 27 percent and Romney at 26 percent. Matthews, the GOP pollster, writes that with Romney supporters largely lined up with Lugar, and Santorum and Mourdock mostly sharing the same support, if Santorum withdraws from the race before Indiana's primary, it could actually hurt Lugar. "If Romney is cruising, they may take a less active role in turning out their supporters and these people are more likely to stay home if they think Romney has the nomination wrapped up," Matthews wrote for Howey Politics Indiana, the newsletter that co-sponsored the poll. Democratic Rep. Joe Donnelly awaits the winner of the GOP primary in the general election, and the poll shows that the competitiveness of the race depends on which Republican advances. Lugar would lead Donnelly by a wide margin, 50 percent to 29 percent. But Donnelly and Mourdock -- both of whom are not well known among Hoosier voters -- would tie, with each candidate earning 35 percent of the vote. "A pollster once told me, 'Joe, if you were at 42 percent a few months out, or you know five, six months out, we'd probably tell you to think of other career options,'" Donnelly told Howey Politics Indiana. President Obama's approval rating in the state is just 39 percent, however, and he trails Romney by nine points in a general-election matchup. That could limit Donnelly's support in the general, even if he faces the lesser-known Mourdock. The poll was conducted by Matthews, the president of Alexandria, Va.-based Bellwether Research, and Democratic pollster Fred Yang, of D.C.-based Garin-Hart-Yang Research Group. The poll consisted of 503 likely general-election voters, surveyed from March 26-27. There were also an oversample to achieve a total of 503 likely GOP primary voters, surveyed from March 26-28. Each sample carries a margin of error of plus-or-minus 4.4 percentage points.